Attunement

Attunement is the stat that allows the use of spells, alongside Intelligence; basically, attunement is how many spells (sorcories, miracles, or pyromancies) that you can stock at once, which you "attune" at a bonfire once you aquire them.

Attunement grants 1 slot at 10, and 5 slots at 19. If you feel like spending 4 more points after that, you can have 6 slots. However, it's recommended that you not spend more afterwords (unless you really want more spells). There are two rings that grant additional attunement slots (one each): These are the "White Seance Ring" and the "Darkmoon Seance Ring." (If I got that wrong, feel free to correct me.)

For the most part, having 5 slots is plenty for stocking your spells. However, if you like having a lot of extra spells to cast, stocking multiples of the same spell will grant extra uses of that spell, at the cost of an attunement slot (two, if you're lucky enough to have 2 copies of a spell of that power, like "Chaos Fire Ball").